1. Field of the Invention
To supply fuel to combustion chambers of internal combustion engines, injection systems for injecting fuel are used whose injectors or valves are exposed to extremely high pressures. Especially valves in the area of Diesel injection technology are conceived such that they can withstand high hydraulic pressures. Such Diesel valves operate on either the inward-opening or the outward-opening principle and have extremely high-speed switching. The function that assures that they remain tight is determined in such Diesel valves in the closed state by minimal leakage and minimal hydraulic forces that act on the valve needle.
If such a valve is used for a fuel injection pump of the distributor type, it serves to provide communication for a pump work chamber with a low-pressure region by way of which fuel is delivered. In this way, the valve controls not only the delivery of fuel to the pump work chamber during an intake but also the end of injection, in which opening the valve prevents further pressure from building up in the pump work chamber.
2. Description of the Prior Art
In Diesel injection valves, it is generally known to minimize the hydrolic forces operative in the opening direction by reducing the pressure engagement faces, which is done by embodying the sealing diameter as equal to the guide diameter of the valve bore.
In the open state, however, such valves are not pressure-compensated, or at least are only partly pressure-compensated. The consequences are that pressure waves which build up upon switching of the valves then generate hydraulic forces on the valve needle. Under some circumstances these undefined forces alter the switching behavior of the valve, which may result in a deviation in the injection quantities.
Moreover, the occurrence of so-called cavitation erosion is well known. If to terminate fuel supply or to determine the fuel injection quantity the valve is opened, a decrease in the pressure in the pump work chamber takes place; previously, this pressure was at a very high level, such as 1000 to 1200 bar, and now via the valve opening fuel flows to the low-pressure region and lowers the pressure in the work chamber. In this outflow, because of the high pressure difference between the high-pressure region and the low-pressure region of the valve, flow separations and so-called flow recirculations can occur. In them, gas or vapor bubbles are formed in the fuel. The cause of this is that the static pressure drops below the vapor pressure of the fuel. As soon as the pressure rises again, namely upon an ensuing implosion, the vapor in the vapor bubbles condenses, and the fuel strikes the valve housing and the valve member at high speed. This effect is all the more pronounced, the more turbulence there is in the flow. Such turbulence occurs especially because of the abrupt increase in the cross section downstream of the valve seat, because there the flow separates under the influence of strong eddy and vapor bubble development.
Especially in the vicinity of the surrounding walls, under some circumstances this can cause material damage, known as cavitation erosion, which occurs especially directly downstream of the valve seat. If this erosion spreads to the valve seat itself, over the long term this leads to malfunctions of the valve.
From the prior art, such as German Patent Disclosure DE 197 17 494 A1, to reduce the danger of cavitation damage it is known to provide a narrowed cross section in the outflow conduit from the chamber located downstream of the valve seat. This kind of narrowed cross section is intended to assure that upon inflow of the medium into the chamber, the medium can rapidly build up a pressure so that possible eddy currents and resultant bubble formation effects are at least reduced. However, the valve known from this reference has the disadvantage that to create this narrowing of diameter in the fuel connecting line that is embodied in the valve housing, additional metal-cutting work steps are necessary, which increase the cost for the production process.
Moreover, from German Patent Disclosure DE 199 40 296 A1, a valve for a fuel injection pump of the distributor type is known in which the cavitation effects are avoided by providing the valve needle with at least one guide face that cooperates, in the open state of the valve, with at least one baffle face provided in the valve bore of the valve housing, in such a way that a flow conduit is embodied that widens at a constant gradient in the flow direction, beginning at a minimal cross section in the region of the valve seat. The effect of this gradient is that the pressure increase is limited to a value that the flow boundary layer at this baffle face can still absorb, without allowing gas bubbles to be released. However, this kind of valve is not capable of completely eliminating the effects of cavitation, and moreover requires complicated, expensive evaluations in terms of flow technology. Accordingly, the valve has a complicated shape which can be achieved only at increased cost and expenditure of time.